An apparatus for the optical recognition of documents is known from U.S. Pat. No. 4,319,137, in which a printed sheet can be recognized based upon distinctive features printed thereon. An extended source of white light illuminates a small strip, which runs transversely across the sheet. The light which is either reflected by the sheet or is transmitted through it is simultaneously being detected by three photosensors. Each photosensor only registers the light from a narrow spectral range, for instance, in the red, green or blue color. For each strip the photosensors transfer three signals corresponding to the three colors to an evaluation system.
German patent document DE-PS 37 05 870 describes a device that can be used as a reading head, which can scan a page line by line. The device includes a row of photodiodes to each of which is assigned a pair of light-emitting-diodes (LED's) which are inclined to each other. Each pair of LED's illuminates the sheet in a region located directly in front of its associated photodiode. A collimator is disposed in front of each photodiode and screens all the light that does not directly originate from the region of the sheet directly in front of the photodiode. The reading head produces a monochromatic raster copy of a printed pattern appearing on the sheet.
It is further known from EP-A 338 123, to create the reading head from a group of interchangeable modules arranged in parallel which include a configuration of rows of photodiodes and light sources that optically scan the sheet in a strip like fashion. Each module operates with light of a predetermined color, and produces the signals associated with a monochromatic raster copy of the printed pattern appearing on the sheet.
Finally, from Swiss patent document CH-PS 573 634, a device is known for scanning a sheet with a single photosensor. In such a device, a small circular area on the sheet is sequentially illuminated by single light sources of different spectral color that are disposed at an angle with the plane of the page, the light sources periodically altering the color of illumination. In synchronism with the cyclic illumination of the area, the single photosensor receives light in the particular spectral region that has been scattered into it in a direction perpendicular to the plane of the sheet. Displacing the sheet after each cycle leads to scanning a small strip on the sheet.
In all the foregoing systems, the disposition of the light sources and photosensors with respect to the plane of the sheet is such that no directly reflected light from the surface of the sheet ever reaches the photosensors. This is a characteristic feature of these systems.